They simply cite the number of voter fraud convictions as the number which actually occur. And this is one of the hardest crimes to solve, since the process is so anonymous.
That's like saying we should alter our crime statistics to include only solved murders, rapes, burglaries, robberies which resulted in convictions to convey our societal ills. Wow! That would be a massive drop.
Voter fraud conviction stats are misleading, most whom are caught plea down to misdemeanors.